In medical diagnostics, for example, computer tomography serves the purpose of producing non-overlapping tomographs. In the case of x-ray computer tomography, these tomographs are calculated with the aid of a computer from data that are recorded during the circular or spiral revolution of an x-ray tube and a suitable detector about the patient (see Kalender, W. A.: Computertomographie. Grundlagen, Gerätetechnologie, Bildqualität, Anwendungen. [Computer tomography. Fundamentals, equipment technology, image quality, applications.] Publicis MCD Verlag, Munich, 2000). The aim is to obtain tomographs in the shortest possible time.
Because of their excessively slow data collection rate, conventional CT devices having a so-called single-row detector, that is to say a detector that has a single detector row or row-type arrangement of detector elements, are not capable of meeting the growing clinical demands (high-resolution recording of complete organs during a pause in breath, large-volume angiographs, three-dimensional representations of anatomical structures with isotropic and high resolution).
Although it would be possible to increase the data collection rate by reducing the time of revolution of the x-ray tube and detector, mechanical limits are soon encountered in so doing. In order, nevertheless, to permit a further increase in the data collection rate, CT devices have recently been developed which have a so-called multirow detector, that is to say a detector that has several rows, for example 4 rows, of detector elements (see Kalender loc. Cit.).
A data collection rate that is further increased by way of so-called area detectors, that is to say multirow detectors with a high number of rows (for example 64 detector rows) is being aimed at, and is the subject matter of the current development.
If a detector channel has defects or is entirely defective, its corrupted or missing signal leads to inconsistencies in the total data volume that have a disadvantageous effect on the quality of the reconstructed tomographs. For example, depending on the number of channels affected and on the type of defect, annular to linear artifacts appear that cover structures in the object being examined. In the most favorable case, they are merely perceived as disturbing by the viewer. But frequently, they influence the diagnosis disadvantageously or even render it entirely impossible.
FIG. 1a illustrates a tomograph of the shoulder area of a human patient recorded by a CT device with an entirely intact detector. If, for example, 64 of 2688 channels of the detector are defective, a tomograph in accordance with FIG. 1b may result. In this case, it is severely affected by artifacts in the case in which the defective channels have a constant signal level in each case independently of the number of x-ray quanta actually impinging on the corresponding detector element. For the sake of simplicity, the term defective channel will always be used below independently of whether a channel is entirely defective or only operating in a defective way.
The cause of a defective channel can be both defects in the actual detector element itself, and defects in the downstream electronic signal processing unit. Consequently, a defect can be eliminated by exchanging the relevant detector element and/or the relevant part of the electronic signal processing unit. Such an exchange is, however, time-consuming and costly.
Methods have therefore been developed that render it possible to restore the signals of defective channels in order thus to be able to dispense with an exchange, or to be able to delay the latter at least until no interruption of the operation of the CT device is necessary. Instead of the tomograph in accordance with FIG. 1b, the application of a correction method known from DE 199 21 763 A1 results in the tomograph in accordance with FIG. 1c which, although clearly having fewer artifacts, is far from being free of artifacts.
In the case of multirow detectors with N rows, the number of the channels is N times the number of the channels present in the case of a single-row detector. In the case of multirow detectors, in particular, however, in the case of area detectors, the probability of a defect therefore rises by at least N times compared with a single-row detector. The economic use of multi-row and area detectors is therefore brought into question because of the high probability of frequently having to exchange detector elements or parts of the electronic signal processing unit.